Endo cyclic

Migration of the double bond from an exocyclic 5-position to an endo-cyclic 4-position occurred on treatment of the 4-mesylate of the 5-enoside (15) with lithium aluminum hydride in ether (22). The structure of... 

The tendency of arsenic and to form additional secondary bonds has interesting consequences in the case of dialkyldithiophosphinate arsocane and stibocane derivatives, X(CH2CH2S)2MS2PR2, X = O, S R = Me, Et or Ph M = As,146 or Sb147,148 The conformational trends and intermolecular association in these compounds are influenced by the competition between endo-cyclic, transannular As X interactions and an exocyclic As- -S secondary interaction. 

Considering enolate 5, where X, Y, and Z stand for three connective points for cyclic enolate formation, the formation can occur between any two of such points, leading to three possible oxygen enolates endo-cyclic 6, 7, and exo-cyclic enolate 8. In such cases, the resident asymmetric center ( ) may be positioned on any connective atom in the substrate. 

Six-Membered Ring (endo-Cyclic). All the previous discussion of stereo-attack is based on steric hindrance, but in the case of a six-membered ring (endo-cyclic) enolate, the direction is affected simultaneously by stereo-electronic effects (Scheme 2-4).10 In the transition state, the attacking electrophiles must obey the principle of maximum overlap of participating orbitals by perpendicularly approaching the plane of atoms that constitute the enolate functional group. Electrophile attacks take place on the two diastereotopic... 

Scheme 2-4. Transition states of the endo-cyclic six-membered ring.

The diastereoselective alkylation reaction of endo-cyclic five-membered ring enolates exhibits good potential for both 1,3- and 1,2-asymmetric induction. In Scheme 2-6, the factor controlling the alkylation transition state is steric rather than stereoelectronic, leading to an auh-induction.13... 

Scheme 2-5. Alkylation of endo-cyclic six-membered ring systems.

There are also many examples of alkylation reactions involving the norbornyl ring system in which the enolate can be either endo- or c.vo-cyclic. Both the endo-cyclic (6, 7) and evo-cyclic (8) enolates exhibit high levels of asymmetric induction due to the rigid ring system. Scheme 2-8 presents some examples for alkylation involving the norbornyl ring system.15... 

Allenic hydrocarbons have often been postulated or observed as reaction intermediates in chemical transformations. In most cases these are cycloallenes of the endo-cyclic type 36 (Scheme 5.3) and they are treated in Chapter 6. 

When the diene used is cydopentadiene, endo-cyclic ring systems result such as are also produced by vegetable cells in camphor and other terpenes. 

Coumarin. The coumarin moiety is found in a number of important drugs. This is a bicyclic heterocycle containing two six-membered rings and two oxygens, one endo-cyclic, one exocyclic. Since the coumarin contains an intramolecular lactone ester, it undergoes hydrolysis to yield a carboxylic acid and a phenol. 

A dissociative first step is anticipated exclusively in photolysis of the endo-cyclic enamide 181 (compare also 80 -a- 85 and 173 -> 174). The reaction is formulated through a ketene intermediate 182. The enamine 183 formed then undergoes hydrolysis to give the oxocarboxylic acid derivative 184 as the product. 

Although the above rationalizations of the rotations for compounds 3, 5, and 7 are inconclusive, most of the evidence seems to indicate that OH/Or (exocyclic) has a smaller numerical value than OH/Or (endo-cyclic). If so, then as suggested by Brewster (4), there is a permolecular effect to rotation arising from the presence of axial substituents at the 1, 2, 4, and 5 positions of the pyranose ring. This problem may be solved... 

The words exo or endo are used to picture if the breaking bond is exo- or endo-cyclic to the smallest ring so formed. 

The C-N bond formation takes place exclusively at the anomeric center because of stereoelectronical effects of the endo cyclic oxygen. 

Asymmetric radical reactions. Several groups have reported asymmetric radical reactions observed with (S,S)- or (R,R)-1 as the chiral auxiliary. Thus the iodide 2 in the presence of BuySnH and AIBN cyclizes mainly to two diastereomeric endo-cyclic products 3 in the ratio 14 1.2... 

Since oxiranes are representative heterocyclic monomers containing an endo-cyclic heteroatom, and the most commonly polymerised of such monomers, they have been subjected to copolymerisations with heterocyclic monomers containing both an endocyclic and an exocyclic heteroatom. Coordination copolymerisations of heterocyclic monomers with different functions are focused on oxirane copolymerisation with cyclic dicarboxylic acid anhydride and cyclic carbonate. However, the statistical copolymerisation of heterocyclic monomers with an endocyclic heteroatom and monomers with both endocyclic and exocyclic heteroatoms have only a limited importance. Also, the block copolymerisation of oxirane with lactone or cyclic dicarboxylic acid anhydride is of interest both from the synthetic and from the mechanistic point of view. Block copolymerisation deserves special interest in terms of the exceptionally wide potential utility of block copolymers obtained from comonomers with various functions. It should be noted, however, that the variety of comonomers that might be subjected to a random, alternating and block polymerisation involving a nucleophilic attack on the coordinating monomer is rather small. 

Pot B type of structures when the substituent is centered under the heading R 1 it is linked to both exo and endo cyclic nitrogens. 

Manganese acetate-promoted oxidative addition of 1,3-dicarbonyl compounds (351) to endo-cyclic enol ethers (352) and enol lactones (353) gives 2,3,3a,6a-tetrahydrofuro[2,3-6]furan derivatives (354) and (355) <87CL223, 91TL711, 91TL7107). 

Both exo- and endo-cyclic alicyclic and heterocyclic ketones have been reduced. Important examples are found in the preparations of cyclo-pentanol (95%), cycloheptanol (92%), /S-pyridylmethylcarbinol (85%), and l-alkyl-4-piperidinols (90%). A comparison of four reagents—sodium and alcohol, lithium aluminum hydride, hydrogen and Raney nickel, and hydrogen and copper-chromium oxide—has been made in the preparation of methyl cyclopropylcarbinol. The last method is superior for the preparation of this compound (SH)%). ... 

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